Mineral nitrogen and microbial dynamics in the forest floor of clearcut or partially harvested successional boreal forest stands

The effects of clearcut and partial harvesting of early-seral trembling aspen plots were compared to conventional clearcut harvesting in mid-seral mixedwood and late-seral conifer plots. Twice a year, for three consecutive years, we assessed mineral N and microbial dynamics in the forest floor of these plots to test three hypotheses related to the higher litter quality of aspen leaves and to the sustained inputs of available C on partially harvested plots: (1) the post-clearcutting mineral N flush and the net [(NO₃^–): (NO₃^– + NH₄⁺)] production ratio (RNI) are higher in aspen plots than in black spruce plots, with intermediate values occurring in mixedwood plots; (2) net N mineralization rates in aspen plots are higher in spring than in autumn; and (3) compared to clearcutting, partial harvesting reduces potential ammonification and nitrification rates. Initial NH₄⁺ and NO₃^– concentrations respectively ranged between 1.7–4.4 and 0.2–1.5 g N kg^–1 N_total, net ammonification and nitrification rates (30 d incubations) respectively ranged between 5.3–17.8 and 0.1–27.6 g N kg^–1 N_total, basal respiration ranged between 20.9–38.9 mg CO₂-C kg^–1 h^–1, and microbial biomass ranged between 6.1–8.7 g C_mic kg^–1. Although clearcutting increased NO₃^– concentrations in aspen plots, the balance of our results did not support our first hypothesis, because NH₄⁺ concentrations increased in conifer plots only, potential ammonification was unaffected by clearcutting, potential nitrification increased in mixedwood plots only, and RNI increased in all plots. In each seral stage, basal respiration, microbial biomass, and metabolic quotient either increased or were unaffected by clearcutting, suggesting that increases in RNI after disturbance were not related to lower microbial immobilisation of NO₃^– due to lower available C. Forest floors in mid-seral mixedwood plots exhibited a distinct combination of mineral N and microbial properties, suggesting that the functional richness of the forest is enhanced not only by the number of species, but also by the diversity of assemblages that are present. Results supported our second hypothesis and showed, furthermore, that net N mineralization in conifer stands is greater in autumn than in spring. Partial harvesting in aspen stands resulted in lower potential mineralization of N and lower RNI, compared to clearcutting. Further lysimetry studies are needed to confirm whether partial harvesting mitigates NO₃^– leaching following disturbance.     

Temperature-dependent nitrogen transformations in acid oak-beech forest litter in the Netherlands

Laboratory incubation experiments have been carried out to quantify net nitrogen mineralization and nitrification in oak-beech litter at temperatures ranging from 0 to 30°C. Net mineralization was linearly proportional to temperature. Nitrification was inhibited at 0,5 and 30°C. As compared with soils under cultivation, there is only restricted knowledge of nitrification kinetics in acid forest litters, especially when temperature is considered. With these litter types, one should be cautious applying high incubation temperatures, which seldomly occur under field conditions.     

森林土壤氮素转换及其对氮沉降的响应

近几十年人类活动向大气中排放的含氮化合物激增 ,并引起大气氮沉降也成比例增加。目前 ,氮沉降的增加使一些森林生态系统结构和功能发生改变 ,甚至衰退。近 2 0 a欧洲和北美有关氮沉降及其对森林生态系统的影响方面的研究较多 ,而我国少有涉及。森林土壤氮素转换是森林生态系统氮素循环的一个重要的组成部分 ,而矿化、硝化和反硝化作用是其核心过程 ,氮沉降作为驱动因子势必改变森林土壤氮素转换速度、方向和通量。根据国外近 2 0 a有关研究 ,首先介绍了森林土壤氮素转换过程和强度 ,论述森林土壤氮素在生态系统氮素循环中的作用 ,然后在此基础上 ,介绍了氮沉降对森林土壤氮素循环的研究途径 ,探讨了氮沉降对森林土壤氮素矿化、硝化和反硝化作用的影响及其机理     

N uptake and N status in ponderosa pine as affected by soil compaction and forest floor removal

Soil compaction and forest floor removal influence fundamental soil processes that control forest productivity and sustainability. We investigated effects of soil compaction and forest floor removal on tree growth, N uptake and N status in ponderosa pine. Factorial combinations of soil compaction (non-compacted and compacted) and forest floor removal (forest floor present and no forest floor) were applied to three different surface soil textures. For studying N uptake, four trees from every treatment were ¹⁵N labeled with 130.6 mg m^–2 of ¹⁵N. Tree responses to compaction were dependent on the forest floor removal level. In loam and clay soils, non-compacted+no forest floor was beneficial to tree growth. Tree growth was depressed with compaction+no forest floor in clay soil. In sandy loam soil, compaction+no forest floor showed the best tree growth. No N deficiency was found in any soil type but a graphical method suggested correlation between N status and tree growth. In loam and clay soils, compaction+forest floor present increased N uptake. Nitrogen uptake was explained significantly by potential N mineralization in loam and clay soils. In sandy loam soil, the effects of compaction and forest floor removal were more complex, with the N uptake improved in the compaction+no forest floor treatment and reduced under compaction+forest floor present. Soil compaction may have influenced N tracer uptake because of improved unsaturated flow and root-soil contact. However, N immobilization may have restricted N uptake in compaction+forest floor present in the sandy loam soil. The study illustrates how soil properties and site preparation can potentially interact to affect N dynamics and forest productivity.     

Effects of fire on soil carbon and nitrogen in a Mediterranean oak forest of Algeria

The effects of wildfire on the dynamics of pH, organic C, total and mineral N and in vitro C and N mineralization were investigated in the soil under oak (Quercus suber L.) trees. Soil samples were taken from 5 to 21 months subsequent to the fire. The pH increased sharply in the burned surface soil (0–5 cm) taken 5 months after the fire and dropped only by half a unit over 14 to 21 months. However, at greater depth (5–15 cm), the burned soil was more acidic than the adjacent unburned soil up to 9 months following the fire, and thereafter its pH rose only slightly above that of the unburned soil. There were sharp rises in the concentration of organic C, total and mineral N in addition toin vitro mineralization activities in the burned surface soil collected 5 months after the fire; these dropped off in the subsequent samples approaching or falling below the values obtained in the unburned surface soil after 21 months. At a depth of 5–15 cm only slight or no increases over unburned soil were evident.     

Nitrogen transformations in two nitrogen saturated forest ecosystems subjected to an experimental decrease in nitrogen deposition

Nitrogen transformations were studied in the forest floor and mineral soil (0–5 cm) of a Douglas fir forest (Pseudotsuga menziesii (Mirb.) Franco.) and a Scots pine forest (Pinus sylvestris L.) in the Netherlands. Curren nitrogen depositions (40 and 56 kg N ha^-1 yr^-1, respectively) were reduced to natural background levels (1–2 kg N ha^-1 yr^-1) by a roof construction. The study concentrated on rates and dynamic properties of nitrogen transformations and their link with the leaching pattern and nitrogen uptake of the vegetation under high and reduced nitrogen deposition levels. Results of an in situ field incubation experiment and laboratory incubations were compared. No effect of the reduced N deposition on nitrogen transformations was found in the Douglas fir forest. In the Scots pine forest, however, during some periods of the year nitrogen transformations were significantly decreased under the low nitrogen deposition level. At low nitrogen inputs a net immobilization occurred during most of the year leading to a very small net mineralization for the whole year. In laboratory and in individual field plots nitrogen transformations were negatively correlated with initial inorganic nitrogen concentrations. Nitrogen budget estimates showed that nitrogen transformations were probably underestimated by the in situ incubation technique. Nevertheless less nitrogen was available for plant uptake and leaching at the low deposition plots.     

Decline of soil nitrogen mineralization and nitrification during forest conversion of evergreen broad-leaved forest to plantations in the subtropical area of Eastern China

We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest: evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation (CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg⁻¹ yr⁻¹) and lowest in CFP (0.4 ± 0.1 mg-N kg⁻¹ yr⁻¹). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg⁻¹ yr⁻¹), but lowest in SS (0.02 ± 0.01 mg-N kg⁻¹ yr⁻¹). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively), soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%, respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass. In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and total C concentration and C:N ratio of forest floor.     

Urbanization and fragmentation have opposing effects on soil nitrogen availability in temperate forest ecosystems

Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co-occurring environmental changes. We examined the effects of urbanization and fragmentation on oak-dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.     

Fine root mass in relation to soil N supply in a cool temperate forest

Soil inorganic nitrogen supply and fine root mass in the top layers of mineral soil (0–5 and 5–10cm) were investigated at upper and lower sites of a cool temperate forest where Fagus crenata and Quercus crispula dominate. At both sites, soil inorganic nitrogen supply was greatest in the 0–5cm layer. The predominant forms of soil inorganic nitrogen supply were NH₄⁺-N at the upper site and NO₃^–-N at the lower site. Fine roots were concentrated in the 0–5cm layer at the upper site, but not at the lower site. The form of supplied soil inorganic nitrogen supply can be important in determining the vertical distribution of fine roots.     

Effect of liming on mineralization of soil nitrogen as measured by plant uptake and nitrogen released during incubation

The effect of lime rates on oat yield and N uptake was measured in a 6-years pot experiment, using 12 acid surface soils (pH 4.7 to 6.0). Mineralization of nitrogen was measured by incubation of soil samples taken after harvest each year from the different lime treatments.Nitrogen uptake was significantly correlated with total N in the soils. Averaged over all 12 soils liming only to pH 7 or above, increased the oat yield significantly. Liming increased the N concentration of grain and the N uptake significantly during a 4-years period, indicating the effect of lime on N mineralization.The mineralization of organic N measured by incubation in the non-limed samples was highly correlated with the total N concentration, but it was not significantly related to the original pH of the soils. The amounts of N released as well as the duration of the lime effect on mineralization varied among soils. When pH was raised to 7 or above, considerable increases in N mineralization occurred in some soils. Based on average values, liming increased N mineralization significantly during a 3-years period. After 3 years, the lime treatments differed only slightly from the non-limed treatments.     

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding ¹⁵ N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH₄ ⁺, NO₃ ⁻, ¹⁵NH₄ ⁺ and ¹⁵ NO₃ ⁻. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization increased. In the H layer, however, both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate ¹⁵N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

长白山两种主要林型下土壤氮矿化速率与温度的关系

在实验室条件下,控制土壤的温度与含水量,测定长白山两种森林类型,阔叶红松林和云冷杉林土壤的氮净矿化速率。将含水量适度和饱和的原样土柱置于5℃、15℃、25℃和35℃恒温箱中培养30d。分析培养前后的NH^ 4-N和NO^-3-N含量,确定土壤有机氮的矿化速率。结果表明,对于不同的土壤和不同的含水量,土壤的净的净矿化速率与温度呈正相关;而净硝化速率在低温段与温度正相关,在高温段则呈负相关,而且整个培养期硝态氮的变化不大。建立两种森林类型土壤氮净矿化速率与温度的一次线性和指数回归方程,发现指数回归效果较好,用回归方程模拟两种森林类型土壤的年矿化量分别为111．8kg/(hm^2.a)和57．4kg/(hm^2.a),与实测值1217．36kg(hm^2.a)和47．41kg/(hm^2.a)很接近。     

Litterflow chemistry and nutrient uptake from the forest floor in northwest Amazonian forest ecosystems

Samples of the fraction of net rainfall passing through the forest floor collected at monthly intervals in four pristine forests in Colombian Amazonia, during the period between 1995–1997 were analysed for solute concentrations to estimate the element fluxes from the forest floor into the mineral soil and root nutrient uptake from these forest floors. Results were compared with inputs by throughfall, stemflow, litterfall and fine root decay. Element concentrations were tested for their relationship with litterflow amounts, rainfall intensity and length of the antecedent dry period and differences in element fluxes between ecosystems were assessed. Concentrations of elements in litterflow followed a similar pattern as those in throughfall, which indicates that element outputs from the forest floor are strongly related to those inputs in throughfall. In the forests studied, the average concentrations of elements as K, Mg, orthoP and the pH of the litterflow decreased relative to that in throughfall in most events, while the concentration of elements such as dissolved organic carbon, H, SO₄ and Si increased in litterflow from these forests. Element concentrations in litterflow showed a poor correlation with variables such as litterflow amounts, rainfall intensity and antecedent dry period, except for K which showed a significant correlation (p>0.95) with analysed variables in all forests. Outputs were significantly different between forests (p>0.95); these fluxes, which particularly concerned cations, being the largest in the flood plain, while for anions outputs increased from the flood plain to the sedimentary plain. After adding the nutrient contributed by litter decomposition and fine root decay, the net outputs of main elements from the forest floors were still smaller than inputs by net precipitation (throughfall+stemflow) indicating that the litter layers clearly acted as a sink for most nutrients. Accordingly, the element balances confirm that the forest floors acted as a sink for nutrients coming in by throughfall, stemflow, litterfall and fine root decomposition. P, Mg and N appeared to be the most limiting nutrients and the forests studied efficiently recycled these nutrients.     

The effects of ammonium sulphate deposition and root sinks on soil solution chemistry in coniferous forest soils

The effects of enhanced (NH_{4 2}SO₄deposition on soil solution cation and anion concentrations and annualionic fluxes were followed using a standardised experimental protocolin six European coniferous forests with contrasting soil types, pollutioninputs and climate. Native soil cores containing a ceramic suction cupwere installed in the field, roofed and watered every two weeks withlocal throughfall or local throughfall with added(NH₄)₂SO₄ at 75 kgNH₄ ⁺-N ha^-1 a^-1. Livingroot systems were established in half of the lysimeters.Untreated throughfall NH₄ ⁺-N deposition at thesites ranged from 3.7 to 29 kg ha^-1 a^-1Soil leachates were collected at two weekly intervalsover 12 months and analysed for volume, andconcentrations of major anions and cations. Increasesin soil solution NO₃ ^- concentrations inresponse to N additions were observed after 4–9months at three sites, whilst one sandy soil with highC:N ratio failed to nitrify under any of thetreatments. Changes in NO₃ ^- concentrationsin soil solution controlled soil solution cationconcentrations in the five nitrifying soils, withAl³⁺ being the dominant cation in the more acidsoils with low base saturation. The acidification responses ofthe soils to the (NH_{4 2}SO₄additions were primarily related to the ability of thesoils to nitrify the added NH₄ ⁺. pH and soiltexture seemed important in controllingNH₄ ⁺ leaching in response to the treatments,with two less acidic, clay/clay loam sites showingalmost total retention of added NH₄ ⁺, whilstnearly 75% of the added N was leached asNH₄ ⁺ at the acid sandy soils. The presenceof living roots significantly reduced soil solutionNO₃ ^- and associated cation concentrations attwo of the six sites. The very different responses of the sixsoils to increased (NH₄)₂SO₄deposition emphasise that the establishment of N critical loadsfor forest soils need to allow for differences in N storagecapacity and nitrification potential.     

Comparison of the litterfall and forest floor organic matter and nitrogen dynamics of upland forest ecosystems in north central Wisconsin

It has been suggested that a feedback exists between the vegetation and soil whereby fertile (vs infertile) sites support species with shorter leaf life spans and higher quality litter which promotes rapid decomposition and higher soil nutrient availability. The objectives of this study were to (1) characterize and compare the C and N dynamics of dominant upland forest ecosystems in north central Wisconsin, (2) compare the nutrient use efficiency (NUE) of these forests, and (3) examine the relationship between NUE and site characteristics. Analyzing data from 24 stands spanning a moisture / nutrient gradient, we found that resource-poor stands transferred less C and N from the vegetation to the forest floor, and that N remained in the forest floor at least four times longer than in more resource-rich stands. Analyzing data by leaf habit, we found that less N was transferred to the forest floor annually via litterfall in conifer stands, and that N remained in the forest floor of these stands nearly three times longer than in hardwood stands. NUE did not differ among forests with different resource availabilities, but was greater for conifers than for hardwoods. Vitousek's (1982) index of nutrient use efficiency (I_NUE1)=leaf litterfall biomass / leaf litterfall N) was most closely correlated to litterfall specific leaf area and percent hardwood leaf area index, suggesting that differences in species composition may have been responsible for the differences in NUE among our stands. NUE2, defined as ANPP / leaf litterfall N, was not closely correlated to any of the site characteristics included in this analysis.     

The effect of acetylene on N transformations in an acid oak-beech soil

The effectiveness of acetylene (C₂H₂) as inhibitor of nitrification was studied in relation to the decomposition of C₂H₂. This was done by examining the effects of single and multiple additions of different C₂H₂ concentrations (10, 100, 1000 Pa) on mineral N and NO₃ ^–-N production in samples of the organic (FH) and upper mineral (Ah) layer of an acid oak-beech forest soil. The decomposition of C₂H₂ was much faster in Ah samples than in FH samples. A single addition of 10 Pa C₂H₂ was not sufficient for complete inhibition of nitrification in the Ah samples. Nitrification was blocked completely by all other C₂H₂ treatments in both FH and Ah samples. Addition of C₂H₂ decreased net mineral N production in Ah samples but not in FH samples. Addition of carboxymethyl-cellulose and chitin to Ah soil had no affect on the rate of decomposition of C₂H₂. Chitin had a negative effect on net NO₃ ^–-N production.     

Calcium content of liming material and its effect on sulphur release in a coniferous forest soil

Soil columns with O + A (Experiment I) or Ohorizons (Experiment II) from a Haplic Podsol wereincubated at 15 °C for 368 and 29 + 106 days,respectively. Three types of liming material differingin Ca²⁺ content, i.e. calcium carbonate(CaCO₃), dolomite (CaMg(CO₃)₂) andmagnesium carbonate (MgCO₃), were mixed into theO horizons in equimolar amounts corresponding to 6000kg of CaCO₃ per ha. In the limed treatments ofExperiment I, the leaching of dissolved organic carbon(DOC) and the net sulphur mineralization (estimated asaccumulated SO ₄ ^2– leaching corrected forchanges in the soil pools of adsorbed and waterextractable SO ₄ ^2– ) increased with decreasingCa²⁺ content of the lime and increasing degree oflime dissolution. In relation to the controltreatment, only the MgCO₃ treatment resulted ina significantly higher net sulphur mineralization. InExperiment I the net sulphur mineralization was 4.06,1.68, 0.57, and 2.14 mg S in the MgCO₃,CaMg(CO₃)₂, CaCO₃ and control treatment,respectively. The accumulated SO ₄ ^2– leachingin Experiment II during the first 29 days was 1.70,0.74 and 0.48 mg S in the MgCO₃,CaMg(CO₃)₂ and control treatment,respectively. In the two experiments there wereconsistently significant positive correlations betweenleached amounts of SO ₄ ^2– and DOC. It wasconcluded that net sulphur mineralization was stronglyconnected to the solubilization of the organic matter(DOC formation) and that pH and/or Ca²⁺ ionsaffected the net sulphur mineralization through theireffects on organic matter solubility.     

四种温带森林土壤氮矿化与硝化时空格局

利用PVC管原位培养连续取样法测定了东北地区4种具有代表性的森林生态系统(硬阔叶林、蒙古栎林、红松林、落叶松林)土壤氮素矿化、硝化的时间动态及氮矿化的空间分布格局.结果表明:4种森林土壤氮素矿化存在明显的时空变异.蒙古栎和红松林土壤在6月份表现出强烈的氮矿化和硝化作用,而硬阔叶林及落叶松林7月份氮素矿化强烈.4种森林生态系统上层土壤的氮净矿(硝)化率显著高于下层土壤.4种林型土壤的硝化过程在氮矿化过程中占有重要地位,其NO-3-N在无机氮中的比例分别为:79.9%～91.1%(硬阔叶林)、50.7%～80.5%(蒙古栎林)、54.1%～92.0%(红松林)、63.7%～86.5%(落叶松林).生态系统构成决定了土壤氮素的矿化能力.阔叶林和针阔混交林生态系统矿化率大于纯针叶林生态系统.硬阔叶林、红松林、蒙古栎林、落叶松林的平均净矿化率分别为:(0.58±0.01) mg · kg-1 · d-1、(0.47±0.19) mg · kg-1 · d-1、(0.39±0.11) mg · kg-1 · d-1和(0.23±0.06) mg · kg-1 · d-1.4种林型氮素矿化作用与地下5 cm温度呈正相关,并受土壤表层 (0～10 cm)水分显著影响.土壤微生物量氮与土壤氮矿化呈显著正相关.     

Controls on leaching from coniferous forest floor microcosms

Summary Studies were conducted with coniferous forest floor microcosms to examine the potential influence of acid precipitation, temperature changes, and plant uptake upon the chemistry of soil leachate solutions. The experimental design included two temperatures and three different simulated throughfall chemistry treatments. When the acidity of throughfall inputs to the microscosms increased, the forest floors exhibited increased leaching losses of calcium, magensium, potassium, and ammonium. The fact that aluminum losses did not incrase correspondingly suggested that there may be a kinetic lag in the mobilization and leaching of aluminum. When microcosms were exposed to warmer temperatures, percolates showed increased leaching losses of calcium, potassium, ammonium, sulfate, nitrate, and organic anions. Forest floor microcosms exposed to simulated average field conditions behaved very much like field plots under the same environmental conditions; however, there were predictable differences in leaching losses between laboratory and field systems for those ions which are strongly controlled by plant uptake. In general, the exclusion of plant uptake from microcosms resulted in increased leaching of potassium, nitrate, ammonium. and sulfate relative to field plots.     

Grass species and soil type effects on microbial biomass and activity

We evaluated plant versus soil type controls on microbial biomass and activity by comparing microbial biomass C, soil respiration, denitrification potential, potential net N mineralization and nitrification in different soils supporting four grass species, and by growing a group of 10 different grass species on the same soil, in two experiments respectively. In the first experiment, none of the microbial variables showed significant variation with grass species while all variables showed significant variation with soil type, likely due to variation in soil texture. In the second experiment, there were few significant differences in microbial biomass C among the 10 grasses but there were significant relationships between variation in microbial biomass C and potential net N mineralization (negative), soil respiration (positive) and denitrification (positive). There was no relationship between microbial biomass C and either plant yield or plant N concentration. The results suggest that 1) soil type is a more important controller of microbial biomass and activity than grass species, 2) that different grass species can create significant, but small and infrequent, differences in microbial biomass and activity in soil, and 3) that plant-induced variation in microbial biomass and activity is caused by variation in labile C input to soil.